A technique by which transistors are manufactured using oxide semiconductors for channel formation regions and the transistors are applied to semiconductor circuits, ICs, electro-optical devices, electronic appliances, and the like has attracted attention.
In particular, an oxide semiconductor having a wide band gap transmits visible light; therefore, the following is attempted: a light-transmitting transistor is manufactured using such an oxide semiconductor with a wide band gap in combination with a gate electrode, a source electrode, and a drain electrode which are formed using a light-transmitting oxide conductor.
For example, Patent Documents 1 and 2 disclose a technique by which a transistor is formed over a substrate having an insulating surface with the use of a semiconductor thin film (with a thickness of about several nanometers to several hundreds of nanometers) including zinc oxide, an In—Ga—Zn—O-based oxide semiconductor, or the like as an embodiment of a transistor using an oxide semiconductor for a channel formation region, and such a transistor is used for a switching element or the like of an image display device.
A transistor using an oxide semiconductor for a channel formation region (also referred to as a channel region) can have a higher field effect mobility than a transistor using amorphous silicon. An oxide semiconductor film can be formed by a sputtering method or the like, and its manufacturing process is easier than that of a transistor using polycrystalline silicon.
Meanwhile, an oxide conductor which transmits visible light and has conductivity is used as a transparent electrode material needed in a display device such as a liquid crystal display. Most oxide conductors which transmit visible light contain metal oxides each having a wide band gap.
As a light-transmitting oxide conductor, an indium oxide-tin oxide alloy (In2O3—SnO2, abbreviated as ITO), zinc oxide, zinc oxide to which aluminum is added (AZO), zinc oxide to which gallium is added (GZO), and the like can be given.
Most of these light-transmitting oxide conductors are oxide semiconductors to each of which an impurity or the like is added. For example, tin, aluminum, and gallium are added to ITO, AZO, and GZO as impurities, respectively.
Further, it is also known that conductivity depends on a deposition condition in the case where the oxide conductor is deposited by a sputtering method. For example, Patent Documents 3 and 4 disclose a technique of forming an oxide conductive layer with high conductivity in a reduced atmosphere containing hydrogen. It is said that when film formation is performed in a reduced atmosphere containing hydrogen, an oxide conductive film containing hydrogen and oxygen vacancy is formed and thus the oxide conductive film has higher conductivity.
Note that Non-Patent Document 1 discloses that a shallow donor level of hydrogen contributes to conductivity of zinc oxide which is an example of an oxide semiconductor having a wide band gap.
In addition, a trend in resolution of a screen of a display device, which is an embodiment of a semiconductor device, is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2160, has been developed rapidly.
As a display device has such a higher definition, a pixel is significantly miniaturized. This tendency is remarkable particularly in middle-sized display devices and small display devices.
In an active matrix semiconductor device in which pixels each provided with a transistor are arranged in matrix, as the pixel is miniaturized, the proportion of the area of the transistor in the pixel is increased; thus, there occurs a problem of reduction in so-called aperture ratio. Therefore, a technique by which the aperture ratio of a pixel of such a semiconductor device is increased with the use of a light-transmitting transistor is expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.
With increase in the number of pixels, writing time for one pixel is shortened, and thus a transistor is required to have high speed operation characteristics, a large on current, and the like. In addition, a problem of energy depletion in recent years has caused a demand for a display device whose power consumption is suppressed. Thus, a transistor is required which is off when the potential of a gate electrode is 0, that is, has so-called normally-off characteristics, and in which an off current is small and unnecessary leakage current is suppressed.
Moreover, a large display device has been developed with a view to a screen size of a diagonal of 60 inches or more or even a screen size of a diagonal of 120 inches or more. Thus, a technique by which increase in wiring resistance due to increase in screen size is suppressed is also required.